Canola is an important agricultural crop in Canada, the United States, Europe and Australia. Weed competition and blackleg disease are significant limiting factors in canola crop production and quality. The challenge for plant scientists has been to develop canola varieties having superior performance with respect to these limiting factors, while at the same time having satisfactory agronomic characteristics, including yield potential, lodging resistance, oil and protein content, and glucosinolate levels that are sufficiently low for registration.
RESISTANCE TO AHAS-INHIBITOR HERBICIDES
Herbicide resistant plants are plants that are able to survive and reproduce following exposure to herbicides at rates of application that would prevent non-herbicide resistant varieties of the same species from surviving and reproducing. Herbicide resistance is particularly important for Brassica, since many weeds, such as stinkweed, shepherd's purse, flixweed, ball mustard, wormseed mustard, hare's ear mustard and common peppergrass have a close genetic relationship with Brassica species. Therefore, it is advantageous for a cultivar to have herbicide resistance not possessed by related weeds.
Some herbicides function by disrupting amino acid biosynthesis in affected species. For example, AHAS-inhibitor herbicides, (also known as ALS-inhibitor herbicides), function by inhibiting the enzyme acetohydroxy acid synthase (AHAS), the first enzyme in the biosynthesis of the amino acids, isoleucine, leucine, and valine. In plants with resistance to an AHAS-inhibitor herbicide, inhibition of the AHAS enzyme is prevented, thus allowing the plant to continue with normal amino acid biosynthesis. Most forms of Brassica are highly susceptible to AHAS-inhibitor herbicides, such as imidazolinones and sulfonylureas.
The development of canola with resistance to imidazolinones, such as PURSUIT™ and ODYSSEY™, was a major breakthrough in weed management technology. The imidazolinones are a family of broad spectrum herbicides which may be applied for in-crop weed control. They control a larger number of problem species than herbicides used in non-herbicide resistant varieties, and offer greater management flexibility, including timing of application and tank mixing. An advantage of imidazolinone (“IMI”) resistant varieties over other herbicide resistant varieties, such as ROUNDUP READY™ (glyphosate) or LIBERTY LINK™ (glufosinate) resistant varieties, is that some imidazolinone herbicides have a soil residual which controls successive weed flushes. This provides a significant advantage to farmers, because it enables them to achieve longer term weed control without a second application of herbicide. Effective weed control increases yield by reducing competition from weed species. It also improves grain quality through the elimination of cruciferous weed seeds. It may also improve weed management in other crops in the rotation, due to reduced weed pressure.
However, a drawback of currently available IMI resistant varieties is that they lack many of the desirable traits found in elite varieties of non-herbicide resistant canola. In particular, none of the currently available IMI resistant varieties have a resistant (“R”) rating to blackleg disease. It is particularly difficult to develop varieties having IMI resistance in combination with other desirable traits because the inheritance of the IMI resistance trait is relatively complex. Unlike the ROUNDUP READY™ trait or LIBERTY LINK™ trait, which are controlled by single transgenes that exhibit complete dominance, the IMI resistance trait is controlled by two unlinked gene pairs having partial dominance. Swanson et al., Plant Cell Reports 7:83-87 (1989) reported the development of imidazolinone herbicide tolerant Brassica napus mutants using microspore mutagenesis. During the process, five fertile double-haploid Brassica napus mutant plants were developed. One of the mutants was tolerant to between 5 and 10 times the recommended field traits of an imidazolinone herbicide. An inheritance study indicated that two semi-dominant unlinked genes combined to produce an F1 with greater tolerance than either of the parents.
Rutledge et al., Mol. Gen. Genet. 229:31-40 (1991) proposed a model for the inheritance of the five AHAS genes in Brassica napus. AHAS2, AHAS3 and AHAS4 appear to be associated with the ‘A’ (rapa) genome and AHAS1 and AHAS5 are likely associated with the ‘C’ (oleracea) genome. AHAS1 and AHAS3 are expressed at all growth stages (Ouellet et al., Plant J. 2:321-330 1992) and mutant forms of AHAS1 and AHAS3 appear to be the most effective tolerance genes. AHAS2 was found to be active only in ovules and seeds. AHAS4 was found to be defective due to interrupted sequences in the middle of the coding region (Rutledge et al., Mol. Gen Genet. 229:31-40 1991) and was not expressed in tissues examined by Ouellet et al., Plant J. 2:321-330 (1992). The last gene, AHAS5, may also be defective (Rutledge et al. Mol. Gen Genet. 229:31-40, 1991). Hattori et al., Can J. Bot: 70:1957-1963, (1992) determined that the DNA sequence of the coding regions for AHAS1 and AHAS3 were 98% identical. DNA sequences of the 5′ and the 3′ ends were also closely related. Few similarities were observed between the sequences of the AHAS2 compared to the AHAS1 or AHAS3 genes.
Thus, there are two known effective mutations for AHAS-inhibitor herbicide resistance—an AHAS1 mutant (believed to be located on the C genome) and an AHAS3 mutant (believed to be located on the A genome). The AHAS3 mutant provides resistance to other AHAS-inhibitor herbicides, such as sulfonylureas. B. napus, B. juncea, and B. rapa all contain the A genome, however only B. napus naturally contains the C genome. The AHAS3 resistance gene alone provides protection under moderate herbicide application rates. Under high herbicide application rates, the AHAS3 mutant shows some injury and delay in maturity. The AHAS1 mutant alone, by contrast, shows severe stunting and is very late to flower and mature at even moderate herbicide application rates. Accordingly, both AHAS1 and AHAS3 mutant genes appear to be required for full resistance. The complexity of the inheritance of the IMI resistant trait results in multiple phenotypes during segregating generations, which presents a significant hurdle to plant breeders. Accordingly, there is a need to develop AHAS-inhibitor herbicide resistant varieties having improved performance characteristics.
BLACKLEG RESISTANCE
Blackleg, caused by the fungus Phoma lingam (Leptosphaeria maculans), is considered the most serious disease of canola worldwide. The fungus causes lesions on the cotyledons, stems, pods, and leaves of canola plants. Stem cankers formed early in the season may girdle the stem, causing lodging and plant death. Less severely affected plants may remain standing but have restricted moisture and nutrient flow, causing the plant to ripen prematurely with shriveled seeds and pods. Infection of canola pods may cause the pods to shatter prior to harvest, resulting in seed loss. Pod infection also results in infection of the seed, which may lead to loss of seed viability. Blackleg is therefore a serious threat to canola yield. Accordingly, plant scientists have invested years of research in attempting to develop canola varieties with superior levels of resistance to blackleg infection.
The Western Canadian Canola/Rapeseed Recommending Committee (WCC/RRC) is a government mandated organization that evaluates canola varieties in Canada and recommends varieties that meet certain criteria for registration by the Canadian Food Inspection Agency. The WCC/RRC uses a classification system to indicate the degree of resistance of canola varieties to blackleg infection. Varieties being tested are compared to the susceptible check variety, Westar, in an inoculated, approved disease screening trial. Testing procedures are set out in a document entitled “Procedures of the Western Canada Canola/Rapeseed Recommending Committee Incorporated for the Evaluation and Recommendation for Registration of Canola/Rapeseed Candidate Cultivars in Western Canada” which is available from the Canola Council of Canada. Canola varieties are placed in one of the following categories, based on a comparison of their disease score with the Westar check variety disease score, as set out below:
DiseaseRatingScore*Classification>90Highly susceptible70 to 90Susceptible50 to 70Moderatelysusceptible30 to 50Moderately resistant<30Resistant (“R”)*Expressed as a percentage of the Westar check variety disease score. 
Canola varieties with a resistant (“R”) rating are considered highly desirable by farmers, due to the decreased risk of damage from blackleg disease. Canola varieties having an “R” rating for blackleg, combined with resistance to an AHAS-inhibitor herbicide such as an imidazolinone, would be particularly desirable. This trait combination would provide improved weed control, while increasing or stabilizing yield by reducing risk of crop loss due to blackleg disease. There are no known canola varieties that provide this advantageous combination of traits. Accordingly, it is an object of the present invention to provide an improved variety of Brassica having an “R” rating for blackleg and resistance to at least one AHAS-inhibitor herbicide.